Multiple volatile variables in a C statement undefined

Undefined behaviour when there are multiple volatile variables accessed
in the one C statement.
Changes to introduce non-volatile temporaries, split statement or make
variable non-volatile.

Author: SparkiDev

src/internal.c

@@ -21208,13 +21208,11 @@ static byte MaskMac(const byte* data, int sz, int macSz, byte* expMac)
     int i, j;
     int r = 0;
     unsigned char mac[WC_MAX_DIGEST_SIZE];
-    volatile int scanStart = sz - 1 - TLS_MAX_PAD_SZ - macSz;
+    int scanStart = sz - 1 - TLS_MAX_PAD_SZ - macSz;
     volatile int macEnd = sz - 1 - data[sz - 1];
-    volatile int macStart = macEnd - macSz;
+    int macStart = macEnd - macSz;
     volatile int maskScanStart;
     volatile int maskMacStart;
-    volatile unsigned char started;
-    volatile unsigned char notEnded;
     unsigned char good = 0;
 
     maskScanStart = ctMaskIntGTE(scanStart, 0);
@@ -21224,22 +21222,31 @@ static byte MaskMac(const byte* data, int sz, int macSz, byte* expMac)
 
     /* Div on Intel has different speeds depending on value.
      * Use a bitwise AND or mod a specific value (converted to mul). */
-    if ((macSz & (macSz - 1)) == 0)
-        r = (macSz - (scanStart - macStart)) & (macSz - 1);
+    if ((macSz & (macSz - 1)) == 0) {
+        r = macSz - scanStart;
+        r += macStart;
+        r &= (macSz - 1);
+    }
 #ifndef NO_SHA
-    else if (macSz == WC_SHA_DIGEST_SIZE)
-        r = (macSz - (scanStart - macStart)) % WC_SHA_DIGEST_SIZE;
+    else if (macSz == WC_SHA_DIGEST_SIZE) {
+        r = macSz - scanStart;
+        r += macStart;
+        r %= WC_SHA_DIGEST_SIZE;
+    }
 #endif
 #ifdef WOLFSSL_SHA384
-    else if (macSz == WC_SHA384_DIGEST_SIZE)
-        r = (macSz - (scanStart - macStart)) % WC_SHA384_DIGEST_SIZE;
+    else if (macSz == WC_SHA384_DIGEST_SIZE) {
+        r = macSz - scanStart;
+        r += macStart;
+        r %= WC_SHA384_DIGEST_SIZE;
+    }
 #endif
 
     XMEMSET(mac, 0, (size_t)(macSz));
     for (i = scanStart; i < sz; i += macSz) {
         for (j = 0; j < macSz && j + i < sz; j++) {
-            started = ctMaskGTE(i + j, macStart);
-            notEnded = ctMaskLT(i + j, macEnd);
+            unsigned char started = ctMaskGTE(i + j, macStart);
+            unsigned char notEnded = ctMaskLT(i + j, macEnd);
             mac[j] |= started & notEnded & data[i + j];
         }
     }

wolfcrypt/src/aes.c

@@ -10350,7 +10350,8 @@ int WARN_UNUSED_RESULT AES_GCM_decrypt_C(
     /* now use res as a mask for constant time return of ret, unless tag
      * mismatch, whereupon AES_GCM_AUTH_E is returned.
      */
-    ret = (ret & ~res) | (res & WC_NO_ERR_TRACE(AES_GCM_AUTH_E));
+    ret = (ret & ~res);
+    ret |= (res & WC_NO_ERR_TRACE(AES_GCM_AUTH_E));
 #endif
     return ret;
 }

wolfcrypt/src/misc.c

@@ -775,7 +775,9 @@ WC_MISC_STATIC WC_INLINE void ctMaskCopy(byte mask, byte* dst, byte* src,
 #if !defined(WOLFSSL_NO_CT_OPS) && !defined(WOLFSSL_NO_CT_MAX_MIN) && \
     defined(WORD64_AVAILABLE)
         volatile word32 gte_mask = (word32)ctMaskWord32GTE(a, b);
-        return (a & ~gte_mask) | (b & gte_mask);
+        word32 r = (a & ~gte_mask);
+        r |= (b & gte_mask);
+        return r;
 #else /* WOLFSSL_NO_CT_OPS */
         return a > b ? b : a;
 #endif /* WOLFSSL_NO_CT_OPS */

wolfcrypt/src/sp_int.c

@@ -18258,22 +18258,22 @@ int sp_to_unsigned_bin_len_ct(const sp_int* a, byte* out, int outSz)
         /* Start at the end of the buffer - least significant byte. */
         int j;
         unsigned int i;
-        volatile sp_int_digit mask = (sp_int_digit)-1;
+        byte mask = (byte)-1;
         sp_int_digit d;
 
         /* Put each digit in. */
         i = 0;
         for (j = outSz - 1; j >= 0; ) {
             unsigned int b;
-            volatile unsigned int notFull = (i < (unsigned int)a->used - 1);
+            volatile byte notFull = ctMaskLT((int)i, (int)a->used - 1);
 
             d = a->dp[i];
             /* Place each byte of a digit into the buffer. */
             for (b = 0; (j >= 0) && (b < SP_WORD_SIZEOF); b++) {
                 out[j--] = (byte)(d & mask);
                 d >>= 8;
             }
-            mask &= (sp_int_digit)(-(int)notFull);
+            mask &= notFull;
             i += (unsigned int)(1 & mask);
         }
     }